Monocrystalline material is used in the manufacture of a wide variety of products, such as electrical circuitry, optical systems, and other microminiature devices. There are a number of methods that are utilized in the growth of a monocrystalline structure from a melt. A synopsis of some of the more significant methods is given below.
The Czochralski method involves the melting of a polycrystalline charge in a crucible by radio frequency induction or resistance heating. A monocrystalline seed is then lowered into the melt while being rotated in a clockwise direction. The crucible and its charge are rotated in a counterclockwise direction. The seed crystal is withdrawn at a slow rate from the melt until the desired diameter of the pulled monocrystalline structure is obtained. The pull speed is increased to maintain the desired diameter of the pull. This procedure continues as long as there is melt remaining in the crucible. One problem encountered with the Czochralski method is that of controlling the cross-sectional area of the crystal. While a circular cross-section is produced, the diameter of the crystal has a tendency to vary widely as the growth proceeds. An additional disadvantage is that the monocrystalline structure pulled from the charge may be contaminated by the material of the crucible.
The Stepanov method (See A. V. Stepanov, Bull. Acad. Sci. USSR, vol. 33 (1969), p. 1775) is a modification of the conventional Czochralski method. By the Stepanov method, a die member is mounted at a fixed position within the crucible such that the upper edges of the die are above the surface of the melt and the bottom of the die is well below the surface of the melt. The key to Stepanov's technique is shaping a melt column, and the melt column shape is used to control the crystal shape. See H. E. LaBelle, Jr., J. Crystal Growth, Vol. 50, 1980, p. 8. A difficulty with the Stepanov method is that constant control of the melt level in the crucible is required, as the level of the melt and hence the shape of the melt column will vary upon formation of the crystal.
In the edge-defined, film-fed growth (EFG) technique, the shape of the crystalline body is determined by the external or edge configuration of the end surface of a forming member or die. An advantage of the process is that bodies of selected shapes such as round tubes or flat ribbons can be produced. The process involves growth on a seed from a liquid film of feed material sandwiched between the growing body and the end surface of the die, with the liquid in the film being continuously replenished from a suitable melt reservoir via one or more capillaries in the die member. By appropriately controlling the pulling speed of the growing body and the temperature of the liquid film, the film can be made to spread (under the influence of the surface tension at its periphery) across the full expanse of the end surface of the die until it reaches the perimeter or perimeters thereof formed by intersection of that surface with the side surface or surfaces of the die. The angle of intersection of the aforesaid surfaces of the die is such relative to the contact angle of the liquid film that the liquid's surface tension will prevent it from overrunning the edge or edges of the die's end surface. The growing body grows to the shape of the film which conforms to the edge configuration of the die's end surface.
The Bridgman-Stockbarger method utilizes an elongated container of material which is melted in a high temperature furnace, after which the container is lowered into a cooler, lower temperature furnace, which allows the material to slowly resolidify as a single crystal. The molten material from which the crystal is grown is completely enclosed during the process, and as a result, strains occur in the material which induce defects when the molten material solidifies.
Float zone refining is another method used to convert polycrystalline material to a high quality monocrystalline rod and, simultaneously, to remove unwanted impurities from the material. In the float zone technique a narrow molten zone is caused to move slowly along the length of a vertically disposed rod of polycrystalline material. As the molten zone moves, the material immediately behind the zone resolidifes as monocrystalline material. The monocrystalline growth is initially nucleated by a single crystal seed and then continues in a self-seeding manner. Impurities in the material tend to congregate in front of the molten zone so that as the molten zone moves, the zone also removes impurities with it, leaving the material behind the zone in a purer state.
In the float zone process with a contactless heater, the molten zone is caused to traverse the length of the polycrystalline rod by moving the rod vertically downward past a stationary heating means such as a radio frequency induction coil that surrounds a material in the contactless manner. In an alternate embodiment of the float zone refining process with a contactless heater, the rod is stationary and the heater moves vertically across the length of the rod. In addition to the translational motion, a rotational motion may also be imparted to improve crystal perfection and uniformity. The float zone process with a contactless heater, while producing a clean monocrystalline result, is very unstable in that the melt zone tends to collapse. Other difficulties in the float zone process with a contactless heater include the presence of a strong thermocapillary flow in the melt that results in banding and the fact that the melt/crystal interface is convex (rather than flat) toward the melt, resulting in solute segregation and dislocations.
The floating-zone process has been used to grow single crystals from a wide variety of materials, including those for electronic and optical applications. It is a containerless process, and for this reason contamination-free single crystals can be grown. This process has also been used extensively for refining. After the melt zone has passed through a sample rod for a number of times, an extremely pure material can be obtained.
The effect of the zone length on the stability of the floating zone is illustrated in FIG. 1. The zone length herein refers to the vertical distance between the melt/solid interfaces at the free surface. Since the melt zone is self-supported by the surface tension of the melt, its stability is rather limited. The stability of the melt zone is, therefore, the most critical issue in floating-zone crystal growth and refining. As illustrated in FIG. 1, an excessive zone length can cause the melt to break out and the melt zone to collapse. The severe distortion of the free surface is a clear indication of poor zone stability.
The stability of the melt zone has been analyzed theoretically by several investigators, based on various simplifying assumptions such as no convection in the melt and flat melt/solid interfaces. Most of these theoretical analyses suggest that the maximum allowable zone length is proportional to (.gamma./.rho.g).sup.1/2, where .gamma. is the surface tension of the melt, .rho. the density of the melt and g the gravitational acceleration.
Coriell et al., J. Crystal Growth, vol. 42, 1977, p. 466, have calculated the free surface shape of the melt zone as a function of the Bond number and the zone aspect ratio. They have also investigated the stability of various interface shapes. Recently, their work has been extended by Riahi et al., J. Crystal Growth, vol. 94, 1989, p. 635, and Lie et al., Physico-Chemical Hydrodynamics, vol. 10, 1988, p. 441, to include the electromagnetic force exerted by an induction-coil heater on the melt zone.
Attempts have been made to improve zone stability. Induction coils have been designed to help levitate the melt zone (e.g., See W. Keller and A. Mubebauer, Floating-Zone Silicon, Marcel Dekker, Inc., New York, 1981). Electromagnetic levitation has also been employed in laboratory experiments. See, e.g., W.G. Pfann, Zone Melting. 2d Edition, John Wiley and Sons, Inc., New York, 1966, p. 116. These techniques, however, are limited to electrically conducting materials. Induction coils have been widely used in the growth of silicon crystals. Due to the exceptionally high .gamma./.rho. ratio of the silicon melt, silicon single crystals of very large diameters, e.g., 75 mm, have been grown, as described in Keller and Mubebauer, supra.